1. Field
The present application relates to a technique for an image analysis of magnification chromatic aberration.
2. Description of the Related Art
Generally, in an electronic camera, it is known that a color shift occurs in imaged image data by magnification chromatic aberration of an imaging optical system thereof. Conventionally, there have been proposed techniques to detect the magnification chromatic aberration from this color shift of image data.
For example, Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2002-344978) obtains magnification chromatic aberration between R and G in image data by finding a minimum point in differences between the R and G components while changing magnification of the R component with various magnification changing ratios.
In addition, Patent Document 2 (Japanese Unexamined Patent Application Publication No. 2002-320237) obtains the magnification chromatic aberration by detecting multiple color shifts in image data and by fitting a mathematical formula model to these color shifts.
Further, Patent Document 3 (Japanese Unexamined Patent Application Publication No. 2004-286684) discloses to fit a linear expression to color shifts in image data in a least square approach.
Usually, a magnification chromatic aberration model including a cubic expression of an image height is used for performing precise fitting of color shifts which changes complicatedly. However, when the cubic magnification chromatic aberration model is used, there arise the following problems.
(1) Easily Affected by a Color Shift Detection Error.
The color shifts detected in image data include detection errors caused by influences of a color structure specific to an object, a color noise, a false color, etc. other than the color shift caused by the magnification chromatic aberration. Meanwhile, the fitting in the cubic expression has a high degree of freedom and easily responds sensitively to these detection errors. Therefore, it may be probable to obtain wrong values for parameters in a mathematical formula model of the magnification chromatic aberration.
(2) The Color Shift Cannot Always be Detected at a Sampling Interval Sufficient for the Image Height.
For performing the fitting correctly on a bending point or the like in the cubic expression, it is necessary to detect the color shifts at a sampling interval appropriate for the image height. In some image data, however, a flat image structure occupies a certain area and the color shifts cannot always be detected in a sufficient sampling interval. When approximation by the cubic expression is performed in this situation, there is a high probability that wrong values are obtained for parameters in the mathematical formula model of the magnification chromatic aberration.
(3) There is a Case in which the Magnification Chromatic Aberration Cannot be Approximated by the Cubic Expression of the Image Height.
In a typical tendency, the color shift by the magnification chromatic aberration is proportional to the image height in a vicinity of an image center and changes nonlinearly against the image height in a peripheral image area. Such a behavior cannot always be approximated by the cubic expression. When approximation by the cubic expression is performed in this situation, there is a high probability that wrong values are obtained for parameters in the mathematical formula model of the magnification chromatic aberration.
(4) There is a Case in which Anisotropic Magnification Chromatic Aberration Occurs
Some lenses may cause the magnification chromatic aberration asymmetric to an optical axis. Meanwhile, the color shift detection errors frequently have anisotropic components. Therefore, it is difficult to discriminate between the anisotropic magnification chromatic aberration and the anisotropic detection errors, and there is a high probability of obtaining wrong values for parameters in the mathematical formula model of the magnification chromatic aberration.